Self-supporting straining device for a continuous digester

ABSTRACT

The present invention relates to a straining device in association with the displacement of liquid through a pulp bed preferably in connection with the withdrawal of cooking liquid in a continuous digester (1) for producing chemical pulp, which straining device (2) is preferably rectangular and comprises a number of strainer-bars (3), arranged in parallel, which have been fixed to at least two rod elements (4, 5), which are placed transversely in relation to the lengthwise extension of the bar-strainers, the said rod elements (4, 5) being made of metal, preferably stainless steel, and having a strength value RP 0.2 at 100° C. which exceeds 200 MPA, preferably 300 MPA, and more preferably 350 MPA, and the width of the said rod element (4) being less than 800 mm, preferably 650 mm, and more preferably 550 mm, so that the straining device becomes self-supporting.

TECHNICAL FIELD

The present invention relates to a straining device in association withthe displacement of liquid through a pulp bed, preferably in connectionwith the withdrawal of cooking liquid in a continuous digester forproducing chemical pulp.

STATE OF THE ART AND PROBLEMS

In connection with producing chemical pulp, environmental considerationshave become increasingly important. In particular with regard to thebleaching of chemically produced pulp, the trend is for rapid movementtowards the use of methods which are ever more environmentally friendly.Many of these so-called environmentally friendly methods require thatthe strength properties of the pulp are very good on entry into thebleaching process. Thus, ever greater demands are made that the digesterhouse process should, as far as possible, spare the fibres containedwithin the pulp. The possibility of providing a mild digester houseprocess is improved by the digester being fitted with efficientstraining devices, thereby permitting efficient displacement of liquidthrough the pulp bed inside the digester.

In particular in connection with using ever higher temperatures in thelower part of the digester (the so-called high-heat zone), it has beenfound to be important to have efficient strainer arrangements, somethingwhich is emphasised by the fact that, in principle, the whole digesteris kept at a mainly constant temperature level, i.e. at so-called"isothermal cooking", with the temperature between the different cookingzones essentially lying preferably within an interval of four (4)degrees C. Consequently, there is a tendency for the number of strainerelements and/or the total strainer area in the lower part of thedigester to increase in prospective digesters. This increase hasinvolved the focus of attention being directed towards the arrangementof individual strainer elements, not least with regard to optimisationof fitting and of maintenance costs.

Nowadays, bar-straining devices are usually employed which comprise anumber of strainer elements, often arranged in a cross-hatch pattern,with each strainer element comprising a number of vertical bars arrangedin parallel, between which bars an opening is formed from which liquidcan be withdrawn. A bar-strainer element which is commonly foundnowadays is constructed (see FIG. 1) by the strainer bars (A), arrangedin parallel, being welded into recesses in flat bars (B) which areplaced transversely. In addition, at the back of these flat bars whichare placed transversely there are welded on supporting pins (C) whichare intended to bear against the inner wall (D) of the digester shell inorder thereby to provide support for the strainer element.

Consequently, a known strainer element of this type is notself-supporting. It has been found that such strainer elements are verysusceptible to incorrect installation, since the principle is based onbearing against the digester shell. If the strainer element isincorrectly installed, so that a gap arises between the wall of thedigester and a/some supporting pin(s), this can lead to the wholestrainer element collapsing. The reason for this is that the strainerelement is very rigid and cannot therefore flex if an extreme loadingshould occur, with the consequent possibility of a "domino effect" ofbroken supporting pins, leading finally to the whole strainercollapsing. A collapsed strainer element is not only a loss per se, butalso involves impaired pulp quality, while at the same time the risk ispresent that, in association with such a collapse, loose supporting pinscan destroy equipment which is located downstream in the process.

In addition, known strainer elements are relatively difficult to cleansince, normally, relatively intricate steps have first to be undertakenin order even to reach the rear surface of the strainer element forwashing it. A further disadvantage of known strainer elements is thatthey have a relatively pronounced tendency to collect accretions,so-called scaling, both on the strainer bars and developing out from thesupporting pins, leading to a decrease in the effective area andincreasing the friction in the strainer elements such that, after acertain time, exceptional measures have to be carried out with a view toeliminating accretions.

SOLUTION AND ADVANTAGES

With the aid of the invention, it has been possible to eliminate theabovementioned problems, the invention comprising a strainer for thedisplacement of liquid through a pulp bed preferably in connection withthe withdrawal of cooking liquid in a continuous digester 1 forproducing chemical pulp, which straining device 2 is preferablyrectangular and comprises a number of bars 3, arranged in parallel,which have been fixed to at least two rod elements 4 which are placedtransversely in relation to the lengthwise extension of thebar-strainers which are made of metal, whereby the said rod elements 4are made of metal with a very good strength, preferably stainless steel,and have a strength value RP 0.2 at 100° C. which exceeds 200 MPA,preferably 300 MPA, and more preferably 350 MPA, and the length of thesaid rod element 4 in a transversal direction in relation to thelengthwise extension of the bar-strainers, is less than 800 mm,preferably 650 mm, and more preferably 550 mm, and the cross-sectionalarea of the said rod elements 4 exceeds 500 mm² and that each endsection of each rod element 4 is intended for interaction with asupporting element 7 so that the straining device becomesself-supporting.

A further advantage of a strainer element according to the invention isthat it is easier to install than the previously used strainer element,while at the same time the element is easier to keep clean, due to itsrelatively modest width. In addition, the risk of blockage is decreasedusing a strainer element according to a preferred embodiment of theinvention, since the number of surfaces placed transversely to thedirection of movement of the pulp bed is further reduced.

A further advantage of a preferred embodiment of a straining deviceaccording to the invention is that, by eliminating supporting pins, therisk of scaling, which can block up the strainer, is diminished. Using aspecial surface treatment, so-called electropolishing, a surface isobtained which produces very low friction and which, moreover, possessesthe advantage of having very little tendency to become coated, i.e.scaling can be further counteracted.

A great advantage of the novel strainer elements is that their width hasbeen optimised with respect to maintenance, so that the strainerelements do not need to be lowered or lifted out of their position inorder to be cleaned. That is to say, the small width means that it ispossible, by using, for example, a high pressure nozzle through acorrectly arranged clean-out door, manually to reach behind the strainerelements and clean them there, while, at the same time, cleaning ofwithdrawal holes, which may have become blocked, can be carried out.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described below against the background of theenclosed figures in which:

FIG. 1 shows essential parts of a previously known strainer element,

FIG. 2 shows essential parts of a preferred strainer element accordingto the invention, in perspective,

FIG. 3 shows the section A--A in FIG. 2,

FIG. 4 shows a front view of a preferred strainer element,

FIG. 5 shows a side view of a strainer element according to FIG. 4 andfinally,

FIG. 6 shows a special strainer element according to the invention whichhas been arranged with a clean-out door.

FIG. 1 shows a known method of arranging a strainer element in which thestrainer element is not self-supporting. This strainer element comprisesa number of strainer bars A which are fixed in parallel into atransverse stay B, in recessed grooves in this transverse stay.Supporting pins, which bear against the inner wall of the digester shellD, are arranged at regular intervals on the rear side of the transversestay B.

FIG. 2 shows a preferred embodiment of a fragment of a digester 1 whichhas been arranged with a strainer element according to the invention.The strainer element 2 comprises a number of bars 3 which are arrangedin parallel in relation to each other. These bars have a height (h)which considerably exceeds the width (b). Due to this design, they canflex laterally but are very flexurally rigid when the force is appliedin the radial direction of the digester, i.e. the direction in whichforces arise in connection with the withdrawal of liquid. In addition,the figure shows that each bar 3 has an inwardly (towards the centre ofthe digester) exposed surface 3A which is greater than the surface 3Bexposed outwards.

The bars 3 are welded to transverse rod elements 4. A fillet weld 8 isarranged flush with the rear surface of the bar 3 and an inwardlydirected plane surface 4A of the said rod element 4. The rod element 4is composed of a material with a very good strength, whereby, in thepreferred case, it applies that the material consists of metal,preferably stainless steel, and has a strength value RP 0.2 at 100° C.which exceeds 200 MPA, preferably 300 MPA, and more preferably 350 MPA.

In addition, it applies that the rods in the preferred case have across-sectional area which exceeds 500 mm², preferably 700 mm², and thatthe flexural rigidity of a part of a straining device selected at willis equivalent to about 4500 mm³.

In addition, it applies that the said rods are arranged at a distanceexceeding 200 mm from each other, preferably at a distance of 230 mm±25mm.

At each end of each bar element 4, the strainer element 2 bears, withoutbeing fixed, against a supporting boss 7 which is welded 9 to thedigester shell 1A. A supporting bar 13, which extends in a verticaldirection, is welded to the boss 7. The function of this supporting baris to provide support for the vertically placed L-bars 10 which form theouter vertical delimitations of each strainer element 2. When, as in thecase shown, two strainer elements 2 are to be installed directlyalongside each other, the supporting bar 13 is welded firmly to themiddle of the boss 7 so that both supporting bar 13 and boss 7 are usedfor installation and force absorption for each of the neighbouringstrainer elements. In order to be able to weld the strainer elements 2firmly in the vertical direction, it is necessary, in this connection,to leave a gap of about 8 mm between neighbouring strainer elements 2.In a corresponding manner, a supporting ring 14 forms, in a horizontaldirection, a support, extending horizontally and continuously, for therear part of a bent flat rod 5 which forms the lower, or upper,respectively, peripheral element of the strainer element 2.

FIG. 3 shows, in cross section, how each individual strainer element 2is arranged in relation to the digester shell, seen from the side. As isevident from this figure, a relatively large gap is formed between thedigester shell 1A and the rear surface 4B of each rod element 4 so thateach strainer element 2 is provided with the possibility of flexingradially outwards. In addition, it is evident from FIG. 3 that each bar3 is, by its end section, fixed into a flat rod 5 which has been bent ina particular manner.

The flat rod 5 has consequently been bent so that it has a first surface5A which is vertically arranged and which at the top is situated on alevel with the inwardly directed surface 3A of the bar-strainers, but atthe bottom is situated somewhat further out towards the digester shell1A. A second surface 5B is directed so that its normal plane forms anangle of approximately 45 degrees (±10°) with the essentially plane,inwardly exposed, surface of the strainer element. In addition, it isdirected obliquely in towards the centre of the strainer element. At thelower end of the strainer element, the corresponding surface 5B isdirected in a corresponding manner so that their normal planes (the 45degrees alternate) meet approximately directly in front of the centre ofthe surface of the strainer element and there form an angle of 90degrees, i.e. at the bottom it is directed obliquely upwards and at thetop it is directed obliquely downwards.

The reason for these surfaces being arranged in such a way is that thisentails an exceedingly favourable design for the pulp bed as it passesthrough this region with regard to the avoidance of blockage. The designeliminates projecting steps which can obstruct parts of the pulp web andlead to blockages. The background for there being a difference betweenthe placement of the upper and lower flat rod 5 with regard to thevertically directed surface 5A, i.e. that the inwardly directed surface5a at the bottom is placed somewhat below (radially outwards) theinwardly directed surface of the strainer element, is that an additionalclearance is thereby obtained as the pulp passes through this point,which further promotes the passage of the pulp bed.

It is additionally evident from FIG. 3 that each strainer element hasbeen fitted, at the vertically extending ends, with L-rods 10 whose oneflange is directed radially (provides stiffening) and whose other flangehas an inwardly directed surface which is situated on a level with theinwardly directed surface of the strainer element, the end section ofthe flange being welded onto the supporting bar 13 in connection withinstallation. It is consequently between the latter end sections that agap is left to allow welding to take place.

In a corresponding manner, therefore, the outer end of the bent flat rodis welded by a fillet weld 18 onto the supporting ring 14. It should benoted that it is not a function of the latter weldings to absorb theloading which acts on the strainer element in a radial directionoutwards, which force is intended to be absorbed by the rod elements 4with the aid of their pivoting disposition against the supporting bosses7. Withdrawal takes place through withdrawal holes 15 in the supportingring 14 down to the header space 16, which is delimited inwards by aplate, and from there onwards through a connection piece in a knownmanner.

FIGS. 4 and 5 show a front view and a side view, respectively, of acomplete strainer element 2 according to a preferred embodiment of theinvention. It is evident in this connection that the strainer element 2has a height which considerably exceeds the width. With regard to thewidth, it applies in this connection that it is, in this preferred case,about 500 mm±50 mm. Furthermore, it applies that the width of the saidrod element 4 is less than 800 mm, preferably 650 mm, and morepreferably 550 mm. The reason for this, as has already been mentioned,is that this design simplifies installation and maintenance and makes itpossible for the element to be "self-supporting".

FIG. 6 shows a special alternative of a strainer element according tothe invention in which a clean-out door 11 has been fitted into thelower section of the strainer element. This clean-out door is arrangedin a plate 12 which has been welded in between lower extensions of theL-bars 10 with a view, as has already been mentioned, to be able to washand clean the strainer element from the rear, as well as to be able toreach withdrawal holes. This special arrangement with a clean-out door11 located at the bottom of strainer elements is intended for strainerarrangements in which the strainer elements 2 are fitted alongside, indirect association with, each other.

Alternatively, the strainer elements are installed in a type ofcross-hatch pattern in which every other position consists of a blankplate and every other position consists of a strainer element. In thecase of this latter alternative, a separate clean-out door can thus bemade in the blank plate area.

The invention is not limited by that which has been shown above, but canbe varied within the scope of the subsequent patent claims. Thus, itwill be evident to the person skilled in the art that the invention isnot limited to the specific forms of strainer element which have beenshown, or to its transverse stays and bar profiles. For example, thetransverse stays can be made to be triangular in shape instead ofrectangular, and the strainer elements made to be square or givenanother suitable configuration which can also deviate from therectangular. In addition, it is evident that bolted joints, or the like,can be used instead of welding for installing the strainer elements.Furthermore, it is evident to the person skilled in the art that thesupporting element can be welded onto the individual strainer elementinstead of being welded onto the digester shell. A disadvantage of thelatter arrangement is that it is not then so easy to compensate forunevennesses in the digester shell. In the preferred case, the procedureis that the supporting rings 14 are first welded on, after which,appropriately with a ruler, it is checked that the distance from theinner surface of the supporting ring to the digester shell isessentially constant in a vertical direction. If a relatively largedeviation should be encountered, this can be compensated by thesupporting boss 7 either being lifted up somewhat in connection withwelding, or by some other adjustment known to the person skilled in theart.

We claim:
 1. A digester having a digester shell and comprising astrainer device; the strainer device comprising a number of strainerelements comprising:a plurality of strainer bars made of metal, saidstrainer bars being arranged in parallel in a lengthwise direction; andat least two rod elements arranged transversely in relation to thelengthwise direction of said strainer bars, said strainer bars beingfixedly attached to said rod elements; wherein said rod elements aremade of metal having a strength value RP 0.2 at 100° C. which exceeds200 MPA, a length of said rod elements in a transversal direction inrelation to the lengthwise extension of said strainer bars is less than800 mm, each of said rod elements has a cross-sectional area thatexceeds 700 m^(m2), and an end section of each rod element is adaptedfor interaction with a supporting element arranged between the digestershell and each strainer element so that the strainer device becomesself-supporting.
 2. A digester as claimed in claim 1, wherein saidsupporting elements are fixed to the digester shell.
 3. A digester asclaimed in claim 1, wherein said supporting elements are arranged atvertically extending sides of the strainer elements, and said supportingelements have a horizontal extension which is adapted to support twostrainer elements installed alongside each other.
 4. A digester asclaimed in claim 1, further comprising a clean-out door arranged inclose association with each strainer element.
 5. A digester as claimedin claim 1, further comprising a clean-out door arranged in a plateinstalled at a bottom of each strainer element.
 6. A strainer as claimedin claim 1, wherein each of said rod elements comprises at least oneplane surface against which said strainer bars are arranged.
 7. Astrainer as claimed in claim 6; wherein said rod elements are secured tosaid strainer bars at a distance between adjacent rod elements exceeding200 mm by means of fillet welding.
 8. A strainer as claimed in claim 1,wherein said rod elements are electropolished.